ddh_intro.md: market to interested users.

Believing this page to be targetted to encouraging people to contribute:

- Explain how instant answers have benefited them as users.
- Make it clear that everyone, regardless of skill level, can provide
  valuable feedback.
- Show that the feedback actually leads to a better experience for everyone.
- Ask them to help.
- Create a consistent hierarchy of changes (better sources -> more
  answers -> coding on answers)